The structure and function of fibrous proteins continues to be studied by various physical-chemical techniques including electron microscopy and gel electrophoresis. Much emphasis is placed on mammalian smooth muscle and particularly on the ultrastructure and peptide composition of recently recognized intermediate filaments of 100 A diameter. The possibility that this filament is a universal fibrous protein will be tested by examining morphologically similar filaments in axons and tissue cultured cells by the same techniques used for smooth muscle. Fine details of the ultrastructure of dense bodies will be studied by electron microscopy and biochemical tools. The similarities of dense bodies and Z lines will be sought. A survey will be made of a wide variety of smooth muscles to determine if the numbers of dense bodies per unit volume of muscle vary from one type to another. If possible correlations will be sought relating structure and function.